A gas recycling photobioreactor was developed to achieve high CO₂ conversion, in which Chlorella vulgaris was cultivated under various light intesities. The light intensity affected the algal growth and the CO₂ concentration in the exit gas. However, the final cell density was independent of light intensity and was limited by nitrate concentration in the medium. In the linear growth phase, the CO₂ concentration in the exit gas raged 4.6 to 6.0% (v/v) when 20% (v/v) CO₂ balanced with 80% (v/v) N₂ was introduced into the photobioreactor. The gas recycling photobioreactor developed in this work was claimed to be a useful system for microalgal CO₂ fixation.